Iraqi forces advance towards Mosul's Old City on Sunday, an offensive to retake the last district in the city still held by the Islamic State. Military commanders say the assault began at dawn after overnight airstrikes by the U.S.-led coalition.

Ahmad Al-Rubaye
/ AFP/Getty Images

Iraqi forces have opened what they hope will be the final assault to retake Mosul, pushing into the the crowded, narrow lanes of the area still occupied by ISIS. The operation, launched at dawn Sunday after a barrage of airstrikes from the U.S.-led coalition, aims to retake Mosul's Old City from the militant group.

The effort marks something of a symbolic return in the Iraqi struggle against ISIS. It was at the Grand Mosque in Mosul's Old City, nearly three years ago, that Islamic State leader Abu Bakr al-Baghdadi declared the caliphate in Iraq and Syria.

Now, the Grand Mosque has become a focal point of fierce fighting, and Baghdadi himself is possibly dead, tentatively believed to have been killed last month in a Russian airstrike on Raqqa, Syria.

But the Iraqi military fears the fighting will only intensify as its troops work to strip the militant group of its final foothold in the country's second-largest city.

"The operation now is about street fighting. Air and artillery strikes will be limited because the area is heavily populated and the buildings fragile," Sabah al-Numan, a spokesman for Iraq's Counter-Terrorism Service, told the news outlet al-Hadath, according to Reuters.

"Conditions in the Old City are desperate," Lise Grande, U.N. deputy special representative and humanitarian coordinator in Iraq, told The Associated Press. She told the wire service about 860,000 people have fled Mosul already.

And, as NPR's Alison Meuse reported, "the U.N. Human Rights Council says it has received credible reports that ISIS increasingly is targeting civilians as they try to flee, leaving more than 230 dead in recent weeks."

In recent days, the U.S.-led coalition has also come in for criticism from some human rights organizations for its use of white phosphorous in the city. The incendiary substance — which was used to create smoke screens for escaping civilians, according to coalition officials — has previously been called an instrument of "indiscriminate attack" by Amnesty International.

"This will be a terrifying time for around 100,000 people still trapped in Mosul's old city and now at risk of being caught up in fierce street fighting to come," Nora Love, acting Iraq director of the International Rescue Committee, said in a statement.

"With its narrow and winding streets," the statement continues, "Iraqi forces will be even more reliant on airstrikes despite the difficulty in identifying civilians sheltering in buildings and the increased risks of civilians being used as human shields by ISIS fighters."

Sunday's push to reclaim the Islamic State's last major stronghold in Iraq comes as U.S.-backed fighters have undertaken a similar effort in Syria, where ISIS militants have long held onto control of Raqqa.

For years, Mosul and Raqqa have stood as the twin pillars of the territory claimed by the Islamic State. Now, Raqqa is encircled — and after eight months of fighting, Iraqi forces are close to wresting back control of Mosul.